Erosion resistant wares composed predominantly of chromium bearing steel

ABSTRACT

Chromium bearing steel wares are rendered resistant to aerodynamic erosion by multiple coating thereof. The outermost coating comprises boron carbide.

United States Patent [1 1 Hordon EROSION RESISTANT WARES COMPOSED PREDOMINANTLY OF CHROMIUM BEARING STEEL [75] Inventor: Monroe J. Hordon, Newton, Mass.

[73] Assignee: National Research Corporation,

Cambridge, Mass.

[22] Filed: 7 Mar. 20, 1972 [21] 1 Appl. No.: 236,164

[52] US. Cl. 29/196.6 [51] Int. Cl B32b 15/00 [58] Field of Search 29/195 A [56] References Cited UNITED STATES PATENTS 2,048,276 7/1936 Marlies 29/195 A Mar. 5, 1974 Primary ExaminerHyland Bizot Attorney, Agent, or Firm-Kenneth W. Brown; Barry R. Blaker; Lawrence A. Chaletsky [57] ABSTRACT Chromium bearing steel wares are rendered resistant to aerodynamic erosion by multiple coating thereof. The outermost coating comprises boron carbide.

6 Claims, No Drawings PREDOMINANTLY OF CHROMIUM BEARING I STEEL FIELD OF THE INVENTION AND PRIOR ART In present aircraft design and manufacture it is often necessaryor desirable to construct certain parts of the engine or airframe from chromium bearing ferro-alloys including, specifically, stainless steels. Accordingly, various leading edge structures for airfoil surfaces such as wings, rotors, vertical and horizontal stabilizers; thrust reversers; nacelle inlets; spoilers; exhaust deflectors; fairings; etc. are often composed of stainless steels. So, too, are various turbine engine parts such as compressor blades and inlet guide vanes. Further, in various chemical processing equipment such as conduits, cyclones,,bag filters and the like it is often found necessary to employ stainless steels'for their characteristic of chemical inertness with respect to many chemicals and corrosive atmospheres.

A problem whichis often encountered in the use of wares such as described above resides in the resistance thereof to aerodynamic erosion, i.e., the deleterious abrasive phenomenon arising when fluid entrained particulate solids are impacted at high velocities against the surfaces of such wares. It has been proposed, of course,to coat such wares with an extremely hard substance such as boron carbide. Unfortunately, such boron carbide coatings, when applied to chromium bearing ferroalloys in thicknesses greater than about microns, are generally found to be severely deficient in adhesion to the substrate and are often quickly spalled and stripped from the substrate under aerodynamic erosion conditions. In accordance with the present invention, however, this adhesion problem has been subas by electron beam or induction heating techniques. Thus, PVD processes are readily distinguishable from chemical vapor deposition coating processes wherein an actual chemical conversion of a precursor material (such as BCI,,) to the ultimate coating (8 C) takes place in the vapor phase at or near thp substrate surface. Further details of PVD coating processes may be had by reference to the following U.S. patent literature: No. 3,579,415, Allen et al., May 18, 1971; No. 3,460,976, Allen, Aug. 12, 1969; No. 3,414,655, Allen, Dec. 12, 1968; No. 3,551,247, Feakes, Dec. 29, 1970. Finally, it is much preferred that the chromium bearing steel ware undergoing the coating method of the invention be heated to above ambient temperature during the PVD coating thereof. Of course, said heating should not be so great as to deleteriously affect the strength of the ware. For the majority of chromium bearing steels, heating to a temperature of between 250 i and 700C during the PVD coating steps will generally stantially ameliorated and highly erosion resistant.

boron carbide coatings are provided on chromium bean'ng steel wares.

OBJECTS. OF THE INVENTION It is a principal object of the invention to provide novel erosion resistant chromium bearing wares.

It is. anotherobject of. the invention to provide novel erosion resistant coatings for chromium bearing wares.'

, GENERAL DESCRIPTION OF THE INVENTION In accordance with the present invention, chromium bearing steel wares are rendered erosion resistant by physical vacuum'vapor deposition of sequential coatings thereon, said coatings comprising, seriatim, (I) nickel or iron, (2) titanium and (3boron carbide.

DETAILED DESCRIPTION, OF THE INVENTION The coatings of the invention are applied by the technique of physical vacuum vapor deposition. (PVD) which technique broadly comprises the condensation .of vapors of the respective coatingmaterials onto the stainless steel substrate under vacuum. Such PVD processes are further characterized by the fact'that the coating materials are vaporized by heating thereof such be found satisfactory. I

Chromium bearing steels suitable for coating by the process of the invention are those ferro-alloys comprising between about 5 and about 20 weight percent chromium. The invention is particularly applicable to wares composed of stainless steels comprising between 1 1 and 16 weight percent chromium. Specific examples of suitable stainless steel materials are: 410, 304, AM- 355, V-57, W-545, CRM-GD, PH 15-7 Mo, Greek Ascoloy, Labelloy C, 1-1-13, 314, AF 71, etc. Complete material balances of the above and other suitable stainless steels may be had by reference to the 1971 SAE Handbook, Society of Automotive Engineers, Inc., New York, N.Y., pages 240243.

The initial nickel or iron coating may be construed as a flash or prime coating and may be relatively extremely thin, in other words, less than 1 micron and preferably from 0.05 to 0.20 micron in thickness. While the initial nickel or iron coating may be ex tremely thin, said coating is nevertheless of extreme importance in providing the improved adhesion characteristics of the overall coatings'of the invention. While there is no intent to be bound by this explanation, it is thought that the nickel or iron coating interacts intimately with the tenacious complex oxides and spinels of the substrate chromium bearing steel surface, thereby to form a firmly adherent bond thereto while concomitantly presenting a fresh, clean and compatible surface to the titanium coating subsequently applied thereto.

The next layer of the triplex PVD coatings of the invention is formed of titanium metal and may have a thickness of from about 0.1 to about 25 microns. Generally speaking, the preferred thickness of the intermediate titanium coating will reside in the range of from about 10 to about 15 microns. The role played by the intermediate titanium metal coating is believed to be that of reduction of deleterious residual mechanical stresses induced by thermal expansion or dilation differences between a chromium based ferro-alloy substrate and a boron carbide coating applied thereto. Accordingly, the titanium coatings forming part of the overall coatings of the invention are believed to act as thermo-mechanical buffers between the chromium bearing steel substrate and the boron carbide exteriormost coating layer in the sense that titanium metal has a thermal coefficient of expansion intermediate that of the steel substrate and the boron carbide outer coating.

strong interfacial bonds with the underlying nickel or iron coating and with the boron carbide coating. in particular, it is known that boron carbide can react to some extent with titanium metal so as to form titanium boride and/or complex titanium-boron carbides. Said reactions are thought to play a beneficial role in providing the markedly improved adherent coatings of the invention.

The outermost layer of the coatings of the invention is composed of boron carbide having a total carbon content of between 15 and 30 atomic percent and preferably between 18 and 25 atomic percent. The thickness of the boron carbide coating can be between about l0gand about 50 microns and is preferably greater than about 20 microns. v

There follows an illustrative, non-limiting example.

Example having a carbon concentration of about 23 atomic percent and the other crucible containing a charge of pure wrought titanium metal. Provision for the initial flash coating of nickel is made by positioning a 6 inch nickel plated tri-filament twisted tungsten wire about 4 inches over the boron carbide containing crucible. The wire is placed in series with an electric power source. The

specimen undergoing the coating operation is cleaned by vapor blasting, placed ina heater equipped specimen holder suspended about 10 inches above the source crucibles and positioned so as to maintain the exposed surface of the specimen substantially parallel to the open ends of the crucibles. The vacuum chamber is closed to the atmosphere, pumped to a pressure of about 1 X, 10 torr and the specimen heated to a temperature of between 400 and 500C. Coating of the specimen isthen undertaken employing the following regimen:

a. a flash coating of nickel is applied by flowing a current of between 0.6 to 1.2 KW power through the nickel coated tungsten wire for a period of about 4 minutes;

b. titanium deposition substantially immediately follows (a) by impinging a 2.4 KW electron beam onto the surface of the titanium charge contained in one of the crucibles for a period of 5 minutes; and substantially immediately after (b) the electron beam serving the titanium source is shut down and the electron beam serving the boron carbide source initiated at a power input of 8 KW and for a period of 10 minutes. Following the above coating procedure the specimen is cooled to about room temperature, in vacuo, the vacuum chamber vented to the atmosphere and the specimen removed fromthe vacuum chamber.

Control specimens were also produced comprising duplex coatings consisting of the titanium and boron carbide coatings only, i.e., without the initial flash coating of nickel. The coating procedures employed for the control coated specimens were essentially the same as outlined above for the production of the coatings of the instant invention with the sole exception that step (a) of the above procedure was not accomplished. Accordingly, the nickel plated tungsten wire was not activated.

Several specimens comprising the triplex coatings of the invention and the duplex Ti/B C control coatings were prepared from sample-plates composed of 410, 304 and AM-355 steels. Type 304 steel is an austenitic nickel/chromium/iron/al'loy steel having a chromium content of about 17l9 weight percent. Type 410 steel is a martensitic chromium steel having a chromium content of about 12.25 weight percent. AM-355 is a semiaustenitic precipitation hardening steel having a chromium content of about 15.5 weight percent. The thicknesses of the coatings applied to the triplex coated specimens were determined to be: nickel coating about 0.1 micron; titanium intermediate coating about 6.5 microns; boron carbide coatings about 40 microns. In the case of the control duplex coated samples the titanium coating had a thickness of about 6.5

microns and the boron carbide coating a thickness of about 30 microns. Attempts to produce boron carbide coatings on the duplex coated specimens of equivalent thickness to those of the triplex coated specimens failed due to immediate lack of adhesion of the duplex coatings on the stainless steel specimens upon cooling thereof in the vacuum chamber. Accordingly, in the case of the control group, it was necessary to reduce the boron carbide coating thickness to the aforementioned 30 micron thickness in order to obtain specimens having reasonably adherent and defect-free coatings. The manner in which the thinner control group boron carbide coatings were produced on the duplex coated specimens was by reducing the time of coating from the original 10 minutes to about 7 minutes.

The resulting coated specimens were subjected to Scotch tape, boiling water immersion and erosion resistance tests. In the so-called Scotch tape test, a r'by 2 inch strip of a commercially available celluloid mending tape is applied to the coated specimen, rolled manually so as to remove all air bubbles and provide a continuous interfacing between the coating and the tape adhesive and allowed to stand for about 1 rninuteJThe tape is then stripped rapidly from the coating and any non-adherent portions of the coating are thereby stripped from the stainless steel substrate. Any amount of the metal carbide coating adhering to the tape is taken as a failure of the test.

The boiling water immersion test is undertaken simply by placing the coated specimens into a boiling demineralized water bath for a period of about 1 hour. Deficient coatings are at least partially removed by this treatment and their absence is readily noted upon removal of the specimens from the bath. Again, substantially any coating removal during the immersion period is construed as a failure of the test.

The accelerated erosion testing of the specimens is carried out by weighing the coated virgin specimens and then subjecting the coated surfaces to controlled sand blasting. in accomplishing this the coated surface of the specimen is held perpendicular to the axis of a boron carbide sand blasting nozzle at a stand-off distance of about 6 inches. The specimen is thenblasted with silicon carbide grit having a particle size population ranging from about 25 to microns for 10-100 seconds. The specimen is then cleaned lightly to remove adherent grit therefrom and re-weighed. The thickness of coating removed from the specimens due f 6 to erosion is calculated from the weight loss, taking into 0. an exteriormost layer of boron carbide bonded to account the exposed surface area of the specimen said intermediate layer, said exteriormost layer under test. In the Table following, said erosion data iS having a total carbon concentration of between expressed in terms of the Erosion Improvement Factor d 3() t i percent d a thick s f betw n which factor represents the ratios of erosion time re- 5 10 and 50 microns quired to remove 25.4 microns of the boron carbide coating compared to the time required to remove 25.4 2. The coated ware of claim 1 wherein the coating microns of a bare stainless steel standard. layer of (a) is nickel and has a thickness of between M TABLE Boiling Stainless Water Erosion Steel Scotch Tape Immersion Improvement Substrate Coating Type Test Test Factor 304 duplex Ti/B c Fail Fail 6.0 triplex Ni/Ti/B C Pass Pass 6.7 4l0 duplex THE, C Pass Fail 3.5 triplex Ni/Ti/B C Pass Pass 4.2 AM-355 duplex Ti/B, C Pass Fail 2.8 triplex Ni/Ti/B C Pass Pass 3.4

What is claimed is: OjOs and 6.20 micron. v 1. An erosion resistant ware composed predomi- 3. The coated ware of claim 1 wherein the intermedinantly of chromium bearing steel having a chromium ate titanium layer of (b) has a thickness of between 10 content of between 5 and weight percent and having and 15 microns. coated over at least that portion of the surface thereof 4. The coated ware of claim 1 wherein the boron caradapted for exposure to an aerodynamic erosion envibide layer of (c) has a thickness of greater than 20 mironment a coating comprising, seriatim: crons.

a. a first metal layer bonded to said surface, said layer 5. The coated ware of claim 1 wherein the boron carhaving a thickness of less than 1 micron and being bide layer of (c) has a total carbon concentration of becomposed of a metal chosen from the group contween 18 and 25 atomic percent. sisting of nickel and iron; 6. The coated ware of claim 1 having the form of a b. an intermediate titanium metal layer bonded to turbine compressor blade.

said first layer and having a thickness of between 0.1 and 25 microns; and *r 

2. The coated ware of claim 1 wherein the coating layer of (a) is nickel and has a thickness of between 0.05 and 0.20 micron.
 3. The coated ware of claim 1 wherein the intermediate titanium layer of (b) has a thickness of between 10 and 15 microns.
 4. The coated ware of claim 1 wherein the boron carbide layer of (c) has a thickness of greater than 20 microns.
 5. The coated ware of claim 1 wherein the boron carbide layer of (c) has a total carbon concentration of between 18 and 25 atomic percent.
 6. The coated ware of claim 1 having the form of a turbine compressor blade. 